1. Field of the Invention
The present invention relates to hand held electrostatic powdered paint spray guns.
2. Description of Related Art
Electrostatic spray guns are typically used to spray electrostatically charged powdered paint onto an object. The object is connected to ground, so that the charged powdered paint becomes attached to the part. Once the object is covered with the powdered paint, the powder is baked until the paint melts and coats the part.
U.S. Pat. No. 3,608,823 issued to Buschor; U.S. Pat. No. 3,731,145 issued to Senny and U.S. Pat. No. 3,819,115 issued to Soderman disclose electrostatic spray guns used to spray powdered paint. A typical electrostatic spray gun has a housing with an internal passage that allows powdered paint to flow through the gun. A hose couples the internal passage to a pressurized paint source which provides dry powdered paint to the gun. The gun also has a nozzle from which the powdered paint is sprayed onto the object. Adjacent to the nozzle are electrodes which induce a charge on the powdered paint as the powder exits the gun. The electrodes are typically connected to a step up transformer and a voltage multiplier which provide voltages in the range of 80,000 to 100,000 volts. The spray guns typically come equipped with a trigger that can close the electrical circuit of the gun, so that the powdered paint becomes charged by the electrode.
Present electrostatic spray guns are connected to a remote stationary power supply that provides a constant voltage to the gun. In operation, the user sets a predetermined voltage level for the gun and begins spraying the object. The exact value of the voltage level of the gun is not critical when the user is spraying a flat surface, because the distance between the surface of the object and the nozzle of the gun is relatively constant.
The range of acceptable voltage levels narrows when the gun is used on curved surfaces, or objects with grooves or indentations. For example, if the voltage level is too high, the magnetic attraction between the charged powdered paint and the object pulls the powder toward the outside surface of the object closest to the gun. Consequently less powder becomes attached to the surfaces within the channels or indentations of the part. Such a phenomenon is typically referred to as the Faraday cage effect.
The power supplies of most commercially available electrostatic spray guns come equipped with a voltage control that can vary the voltage supplied to the gun. The console of the power supply typically has a knob or other control switch that allows the user to change the voltage to the gun. When a Faraday cage effect occurs, the user can go over to the console and turn down the voltage level of the power supply. Unfortunately this can be a time consuming process, sometimes resulting in two or more trips to the console, until a voltage level is found that will allow the powdered paint to reach into the crevices or indentations of the part. If the parts are moving on a conveyor system, the operator may not have time to continually walk back to the console. It would therefore be desirable to have an electrostatic spray gun which allows the user to efficiently vary the voltage of the gun without having to operate the console of the power supply. Additionally, it would also be desirable to have an electrostatic spray gun system that varies the powder density supplied to the gun as a function of the voltage provided to the gun electrodes, so that an optimum voltage to powder ratio is maintained during the spray cycle of the system.